The present invention relates to a process for producing a poly-silicon (hereinafter abbreviated as poly-Si) film for liquid crystals and semiconductor devices, and a method for inspecting the poly-silicon film.
The reason why a poly-silicon film is superior to an amorphous silicon (a-Si) film as the active layer of a thin film transistor (TFT) used as a driver element in a liquid crystal display is as follows: in the case of the poly-silicon film, since the mobility of a carrier (electrons in n channel or holes in p channel) is high, the cell size can be reduced, so that the precision and minuteness of the liquid crystal display can be enhanced. In addition, the formation of a conventional poly-Si TFT requires a high-temperature process at 1,000xc2x0 C. or higher. On the other hand, a TFT having a high carrier mobility can be formed in a low-temperature process permitting employment of an inexpensive glass substrate, when there is adopted a low-temperature poly-silicon formation technique in which annealing of only a silicon layer with a laser does not make the temperature of the substrate high.
In this laser annealing, as shown in FIG. 13, an a-Si film formed on a glass substrate is scanned while being irradiated with light absorbable thereby, to make the whole a-Si film into a polycrystal, whereby a poly-Si film is obtained. As shown in FIG. 14, the poly-Si grain size varies with the surface density of irradiation energy (fluence) of a laser, so that the stability of the laser reflects on the grain size distribution of the poly-Si. The carrier mobility of the poly-Si film increases with an increase of the grain size. In order to attain high TFT characteristics with in-plane uniformity, it is necessary to make the grain size distribution uniform and maintain a large grain size. To attain the large grain size, employment of a fluence in the D region shown in FIG. 14 is sufficient. However, if the fluence shifts upward owing to the instability of the laser, or the like, the fluence enters a region shown as the E region in FIG. 14, i.e., a region where the poly-Si film contains micro crystals with a grain size of 200 nm or less. In this case, the carrier mobility is decreased, resulting in a faulty device. The grain size varies not only with the laser fluence but also with the nonuniformity of thickness of the a-Si film before the laser annealing. Therefore, in order to form the poly-Si film so that its grain size may always be in a definite range, the laser instability and the thickness change of the substrate have to be kept slight. For this purpose, control of the grain size is necessary. Accordingly, it becomes important to control the poly-Si grain size to keep it constant, by checking the poly-Si grain size and feeding back the check result to the laser annealing conditions.
As a method for the control, measuring the grain size itself of the poly-Si is the most reliable. The grain size has been measured by incorporating a sample for the check into an initial or intermediate production lot, or by randomly sampling a product and directly observing the grain size of a poly-Si film formed in a production process, by an electron microscope or a scanning tunnel microscope. As other prior arts, there are the following methods. Japanese Patent Kokai No. 10-214869 discloses a method in which a poly-silicon film is evaluated on the basis of its transmittance. According to this method, the grain size cannot be estimated, though insufficient crystallization due to the insufficient fluence of laser beams can be monitored on the basis of the ratio between a-Si and a poly-Si by utilizing the difference in absorption coefficient between a-Si and the poly-Si. Japanese Patent Kokai No. 11-274078 discloses a method in which a poly-silicon film is evaluated on the basis of its surface gloss (reflectance). In this method, the change of the gloss with the poly-Si grain size is utilized and the gloss is considered to be minimal at an optimum poly-Si grain size. This optimum poly-Si grain size corresponds to a grain size at which the reflectance becomes minimal, namely, the surface roughness becomes maximal.
The pressure resistance of the gate insulating film of a device becomes insufficient if the surface roughness of the film is high. Thus, a grain size detected by the utilization of conditions under which the surface roughness becomes maximal is used in a method in which there is detected a region where the risk of insufficient pressure resistance due to a remarkable surface unevenness is the highest. If this region is employed, a process for reducing the surface roughness is required, resulting in a complicated production process. Thus, a device production process dependent on the above-mentioned prior art substrate examination methods requires a special process for reducing the surface roughness, and its adoption is limited to that at a grain size (about 300 nm) in the B region shown in FIG. 14. However, a poly-Si film having a higher carrier mobility has to be formed in order to produce a liquid crystal which consumes less electricity and has higher precision and minuteness. To form such a poli-Si film, it is sufficient that there is employed the D region shown in FIG. 14, i.e., a region in which the grain size becomes maximal. For this purpose, it is necessary to estimate the grain size, independent of the surface roughness. As a method for determining the D region, the above prior arts are not suitable and examination by electron-microscopic observation is not suitable for determination on the site of a mass production line because it requires human labor and a long time for obtaining a measurement result. Accordingly, it is difficult to produce stably a poly-Si substrate having a low surface roughness and a grain size of more than 300 nm. The present invention was made in view of the above problems, and makes it possible to determine a region where the surface roughness is low and the grain size of a poly-Si is maximal, by a simple method. Thus, the present invention is intended to provide a process for producing a poly-Si film having a low surface roughness and a high carrier mobility, without product nonuniformity or in high yield.
For the achievement of the above object, the present invention provides a process for producing a poly-Si film which comprises a step of forming a poly-Si film by annealing a silicon film set on a substrate, by light irradiation, a step of measuring a light diffraction pattern of the poly-Si film, and a step of selecting the poly-Si film on the basis of the light diffraction pattern.
The aforesaid silicon film is composed of an a-Si film and is converted to a poly-Si film by annealing by laser beam irradiation. The grain size of the poly-Si film is estimated by measuring the angular distribution of scattered light intensities, and the quality of the poly-Si film is judged by knowing whether its grain size is in the range of the upper limit of the average grain size to the lower limit which range is defined by the relationship between the field-effect mobility and the grain size.
As shown in FIG. 1, a light source 2 used for the above-mentioned poly-silicon size measurement with angle dependency of scattered light intensity is a laser having an output wavelength of 540 nm or less and emits laser beams perpendicularly to a substrate 1 having the above-mentioned poly-Si film formed thereon. A plurality of light detector units 7 are located at their respective angles in a range of about 5xc2x0 to about 45xc2x0 in order to measure the angular distribution of the intensities of scattered lights from the irradiation region. As shown in FIG. 7, the relationship between the poly-Si grain size and the breadth of angular distribution of scattered light intensities in the light diffraction pattern of the poly-Si film is explainable in terms of a relation based on Fourier transformation which is such that in general, the breadth of angular distribution of the intensities of scattered light from particles decreases with an increase of the particle size. FIG. 7 shows both the case of single particles not interfering with one another and the case of densely aggregated particles interfering with one another. In the latter case, the distribution is such that the scattered light intensity decays at a scattering angle close to zero. In either case, when distribution A with a larger breadth of angular distribution and distribution B with a smaller breadth of angular distribution are compared for the grain size, the grain size in the case of distribution B can be judged to be larger than in the case of distribution A. According to this principle, the grain size is measured without destruction.
In the above process for producing a poly-Si film, there is measured the breadth of angular distribution of scattered light intensities in a light diffraction pattern of a poly-Si formed as a thin film by irradiating a-Si with exciter laser beams, in the production procedure of a poly-Si. From the measurement result, the grain size of the poly-Si is estimated. On the basis of the estimation result, the fluence of anneal laser beams is set. When the fluence of anneal laser beams is too low, the grain size does not become sufficiently large. Therefore, the lower limit of the fluence is fixed.
On the other hand, in a region where micro crystals are formed as shown in FIG. 17 because of too high a fluence, the average grain size is decreased and a linear pattern appears in a light diffraction pattern as shown in FIG. 18. Micro-crystal streak lines are detected by detecting the linear pattern. The upper limit of the fluence of anneal laser beams is fixed so that the micro crystal streak lines may not appear. The lower limit and upper limit of the laser fluence are fixed as follow in the range of control of the average grain size.
The range of control of the average grain size (the upper limit and lower limit of the average grain size) is determined from a desirable field effect mobility and the range of variation of in-plane distribution of field effect mobility by utilizing the relationship between the average grain size and the field effect mobility shown in FIG. 15.
In order to determine the laser annealing conditions before the production, annealing is conducted under laser fluence conditions stepwise varied in a substrate, after which the average grain size is estimated from the breadth of angular distribution in a light diffraction pattern of the resulting poly-silicon film, and the laser annealing conditions are determined so that the average grain size may be in the range of control. In an actual process, the reduction of the product nonuniformity and the improvement of the yield are carried out by estimating the in-plane distribution of the grain size of a poly-Si film after laser annealing, judging the quality of the substrate sample obtained by the laser annealing, according to the above-mentioned criterion, and sending the sample to a subsequent step only when it is judged good. In this case, a total inspection need not always be carried out, and either a sampling inspection or a total inspection may be chosen depending on the range of variation of the average grain size of each substrate sample in one and the same lot. That is, when the range of variation of the average grain size of each substrate sample in one and the same lot is in a range of xc2x120%, inspection of at least one substrate sample in one and the same lot is sufficient. In a conventional sampling inspection, three samples, i.e., the first sample, an intermediate sample and the last sample in each lot are inspected. When the range of in-plane variation of the average grain size is in a range of xc2x120% for all of the three samples, the whole lot is considered as a good lot. However, if the range of variation of the average grain size of even only one of the three substrate samples is outside the range of xc2x120%, the sampling inspection for the lot is switched over to a total inspection.
Thus, substrate samples are screened by the total inspection or the sampling inspection. According to the data shown in FIG. 15, by controlling the grain size so that the average grain size may be 500 nm or more and that the range of variation of the average grain size in the in-plane distribution of average grain size values may be in a range of xc2x120%, there is formed a poly-silicon film having a field effect mobility not less than a set value 200 cm2/VS and an in-plane variation of field effect mobility in a range of xc2x110%.